WASHINGTON -- Don't worry too much, for now, about rising seas caused by melting ice in Greenland and Antarctica. The big threat this century could come from small thawing glaciers, researchers reported Thursday.

Even though these glaciers contain only 1 percent of the water tied up in the great ice sheets of Antarctica and Greenland, they could account for 60 percent of an anticipated rise in the world's sea level by the year 2100.

Sea-level rise is seen a key consequence of global warming, and much of the concern has focused on the big ice sheets that contain the vast majority of the world's ice.

Researchers writing in the online journal Science Express estimate melting glaciers, which are located all over the globe including in the tropics, could add between 4 and 10 inches to world sea level this century.

While this may not sound like much, consider that some 100 million people live within 3.3 vertical feet of sea level, said Mark Meier of the University of Colorado-Boulder, a lead author of the study.

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"If we had almost a foot (of sea-level rise) just due to the small glaciers, add that to the amount due to the ice sheets, which could be appreciable by 2100, and add to that the ocean warming which will cause it to expand in volume, then we get a rise that we can't ignore," Meier said in a telephone interview.

A VAST INLAND INCURSION

Even a tiny amount of sea-level rise can make a vast inland incursion of water in flat coastal areas, as much or more than 100 times the distance inland as the height of the rise, he said.

Meier said the huge amounts of ice locked in Greenland and Antarctica hold the potential for "some really horrendous sea level rise" -- as much as 3.3 feet -- if they ever completely melt.

That is unlikely to happen this century, although Greenland's ice sheet currently contributes 28 percent and Antarctica's contributes 12 percent to the total ice-melt that fuels sea-level rise, the researchers found.

"Now don't ask me about 1,000 years from now," Meier said. "But for the next few generations we think that we should not ignore the little glaciers."

There are hundreds of thousands of small glaciers all over the world, including in tropical New Guinea, but the important ones in terms of global sea-level change are in Alaska, Canada, Russia and Scandinavia, Meier said.

Part of the reason glaciers are contributing more to rising seas is because of rapid changes in how they flow, co-author Robert Anderson said in a statement.

Many glaciers are getting thinner and that makes them slide more quickly toward the sea.

"While this is a dynamic, complex process and does not seem to be a direct result of climate warming, it is likely that climate acts as a trigger to set off this dramatic response," said Anderson, also of the University of Colorado-Boulder.

The sea ice that seasonally covers the Arctic Ocean would contribute nothing to sea-level rise, much as a melting ice cube in a glass of water would not make the glass overflow. Rising seas are caused by water from ice that has been locked up on land.